In general, since many impacts may be applied to wheels of a vehicle depending on road surface conditions, a suspension apparatus is installed to absorb shocks such that the impacts cannot be directly transmitted to a vehicle body.
A ball joint, which can be rotated and inclined in an arbitrary plane, is generally used in a steering or suspension system of a vehicle.
Such a ball joint is configured such that a spherical head part of a ball stud is inserted into a ball seat to be rotated or inclined in the ball seat, the ball seat is fixed by a housing, and a dust cover is fixed between ends between the housing and the ball seat.
In order to simplify a manufacturing process, such a ball joint is formed through a well known method of integrally injection-molding a ball seat formed of a synthetic resin material at the outside of a spherical head part of a ball stud.
FIG. 1 is a view showing a state in which a conventional ball stud and a conventional ball seat are integrally injection-molded, and FIG. 2 is a view showing a state in which grease is applied after the ball stud and the ball seat are integrally injection-molded.
Referring to FIG. 1, a ball stud 10, and a ball seat 20 formed of a synthetic resin material, which constitute a ball joint, are provided. The ball stud 10 has a spherical head part 11 that can be rotated and inclined in the ball seat 20.
The ball seat 20 is integrally formed outside the spherical head part 11 of the ball stud 10 through injection molding. After the injection molding, cooling is performed. Upon cooling, a volume of the ball seat 20 formed of a synthetic resin material is reduced in a direction of the spherical head part 11, and thus, a tightening force applied to the spherical head part 11 is increased after the cooling, increasing a torque.
In addition, grease is applied between an outer circumference of the spherical head part 11 and an inner circumference 20a of the ball seat 20 to secure smooth rotation therebetween. For this, as shown in FIG. 2, after the grease is applied to an upper end of the spherical head part 11 after the cooling, the ball stud 10 is rotated and rolled to fill the grease between the outer circumference of the spherical head part 11 and the inner circumference 20a of the ball seat 20. However, as described above, due to contraction of the ball seat 20 upon cooling, a space between the outer circumference of the spherical head part 11 and the inner circumference 20a of the ball seat 20 is narrowed such that the space cannot be filled with the grease. Accordingly, a torque generated between the ball stud 10 and the ball seat 20 cannot be easily reduced.